Experimental and Monte Carlo dosimetry of the Henschke applicator for high dose-rate ¹⁹²Ir remote afterloading

We have performed extensive computational and experimental dosimetry of the Henschke applicator with respect to high dose-rate ¹⁹²Ir brachytherapy using a GAMMAMED remote afterloader. Our goal was to generate clinically useful two- and three-dimensional look-up tables. Dose measurements of the Henschke applicator involved using TLD chips placed in a polystyrene phantom. Monte Carlo simulations were performed using the MCNP code. The computational models included the detailed geometry of ¹⁹²Ir source, tandem tube, and shielded ovoid. The measured dose rates were corrected for the dependence of TLD sensitivity on the distance of measurement points from the source. Transit dose delivered during source extension to and retraction from a given dwell position was estimated by Monte Carlo simulations, and a correction was applied to the experimental values. For the applicator tandem, the ratio of dose rates obtained by MCNP to those measured by TLD chips ranges from 0.92 to 1.10 with an average of 0.98 and a standard deviation of 0.02. The measured and calculated dose rates at 1 cm on the transverse axis are 1.10 cGy U^-1 h^-1. For the shielded ovoid, the ratio ranges from 0.88 to 1.16 with an average of 1.00 and a standard deviation of 0.07. Causes of the discrepancy between the Monte Carlo and TLD results were identified. We found that the combined uncertainty of measured dose rates due to these causes is 5.6% for the applicator tandem and 8.4% for the shielded ovoid. Therefore, the results of the Monte Carlo simulation are considered to have been validated by the measurements within the uncertainty involved in the calculation and measurements.